Sexually transmitted infections (STIs) are caused by more than 25 infectious pathogens that can be transmitted through sexual contact. STIs include diseases like chlamydia, gonorrhea, hepatitis B, herpes, HIV/AIDS, and syphilis. While some STIs may only cause mild symptoms, left untreated some STIs can lead to serious long-term health problems like infertility, cervical cancer, and even death. It is important for sexually active people to practice safe sex and get tested regularly to prevent the spread of STIs.